The present invention relates to a method for measuring the surface height of a material bed conducted on a conveyor belt to continuously operated thermal treatment, continuously operated sintering, essentially throughout the whole width of the conveyor belt.
In continuously operated sintering, there is nowadays generally used a conveyor-type sintering device, where on the conveyor belt, there is first formed a material bed. The material bed usually consists of spherical, fragile pellets, which by means of a high-temperature treatment, i.e. sintering, are hardened so that the pellets can be fed to further treatment, for instance to a smelting furnace. In the sintering of a material bed, through the material bed located on the conveyor belt, and simultaneously through the conveyor belt itself, there is first conducted hot gas, so that the temperature of the material bed rises, for instance in the case of ferroalloy pellets, up to the range of 1300-1600xc2x0 C. At a high temperature, the fragile pellets react with the hot gas and are hardened in the process. Thereafter the hardened pellets obtained in the material bed are cooled by conducting cooling gas through the material bed and the conveyor belt. Thus the conveyor belt employed in continuously operated sintering is used under remarkable fluctuations of temperature.
In order to obtain an advantageous sintering product, the material bed must be essentially even throughout the whole width of the conveyor belt. For measuring the evenness of the material bed, there are used for instance wires that are spaced apart over the conveyor belt, said wires being at one end connected to a common bar installed in parallel to the conveyor belt, so that the wires move along the top surface of the material bed. Moreover, in the measuring device there arc installed separate bars for the top surface of the material bed, said bars defining the allowed lower and upper limits of the material bed. If the position of the bar connected to the measuring wires falls outside the allowed position of the top and bottom limits, there is sent an emergency notice in order to be able to adjust the material bed to the desired level before the sintering process.
For measuring the surface height of material beds, there also are used optical measurement devices installed at the edge of the conveyor belt. In a similar way, as the wires described above, said optical measurement devices measure only the top surface of the material bed with respect to the proceeding direction of the conveyor belt. By means of this equipment, it is not, however, possible to detect for instance potholes or apertures made in the material bed, i.e. spots where the amount of material to be sintered is slight or nonexistent. Such potholes or apertures may cause even large damages to the conveyor belt, because normally the conveyor belt as such is resistant to a temperature of about 500xc2x0 C. only, whereas in the material bed sintering zone, the temperature in the top part of the material bed may rise up to 1300xc2x0 C. and over.
The object of the present invention is to eliminate some of the drawbacks of the prior art and to realize an improved method for measuring the surface height of a material bed going to a continuously operated thermal treatment, i.e. to continuously operated sintering, so that the height of the material bed can be measured essentially along the whole width of the conveyor belt. The essential novel features of the invention are apparent from the appended claims.
According to the invention, in order to measure the surface height of the material bed in a continuously operated thermal treatment of said material bed, i.e. in continuously operated sintering, above the conveyor belt used for transporting the material bed through the various thermal treatment steps, essentially immediately after feeding the material bed onto the conveyor belt in the proceeding direction of said conveyor belt, there is installed at least one light source emitting an essentially narrow, linear light beam, which is used for illuminating the material bed composed of single spherical and fragile particles essentially along the whole width of the material bed. Now the beam of light forms on the material bed surface a line representing the height of the material bed; In order to detect the line cast by the light source, essentially above the line formation spot, there is installed at least one camera, by means of which the line representing the material bed surface is transmitted to at least one image processing arrangement. The image of the material bed, obtained by the image processing arrangement, is compared with a desired, predetermined and essentially advantageous surface height image of the material bed. The image processing arrangement is further connected to an arrangement whereby it is possible, when necessary, to adjust for instance the speed of the conveyor belt in order to adjust the surface height of the material bed to a desired value.
In a preferred embodiment of the invention, the employed light source is a laser light source that is installed above the conveyor belt transporting the material bed, so that the beam of light emitted from the light source meets the material bed surface at a sharp angle. With respect to the conveyor belt, the light source is installed so that the light source meets the material bed essentially near to the spot where the material bed is fed onto the conveyor belt. The angle between the light beam and the surface of the material bed is within the range 0-45 degrees, advantageously 30-40 degrees. However, the light source can also be installed, in relation to the material bed surface, so that the light beam coming from the light source forms, with respect to the proceeding direction of the conveyor belt, an essentially right angle, in which case the light beam meets the material bed essentially perpendicularly.
In order to detect the line formed by the light beam on the surface of the material bed, there is advantageously installed a camera above the line formation spot, and the image created by said camera is transmitted to an image processing arrangement. In the image processing arrangement, the image is divided into sections, essentially so that a given section of the image corresponds to a given part of the line created on the surface of the material bed in the transversal direction of the conveyor belt. The obtained image is then processed so that each section is represented by a respective column on the display screen. In height, each column corresponds to the height of the material bed located on the conveyor belt. The obtained image is compared with an image created by a the predetermined, desired surface height. In case the image obtained of the material bed deviates in some respects from the desired image of the material bed, an emergency notice is given, and on the basis of said notice, the material bed is adjusted to be suitable, for instance by adjusting the speed of the conveyor belt prior to feeding the material bed to the various steps of the thermal treatment. The adjustment of the conveyor belt speed functions as an adjustment of the material bed height; for example when the need arises to raise the material bed height, the speed of the conveyor belt is slowed down, so that at an even feed rate, more material at a time is fed onto the conveyor belt.
When applying the method according to the invention for measuring the surface height of a material bed that is subjected to thermal treatment, the measurement as such can be carried out without mechanically touching the material bed itself. This is particularly advantageous, because normally the material bed is formed of fragile, moist pellets, which are easily broken by mechanical contact. Moreover, when operating according to the method of the invention, the measuring device is free of any mechanically wearing parts. Further, when the method according to the invention is applied, in the measuring of the surface heights of material beds, there are not needed any such measuring devices that could present an obstacle for a smoothing out of the material bed prior to the thermal treatment thereof.